The instant invention relates to a damping device for use in combination with a fluid pressure measuring apparatus-transducer system used for measurement of circulatory pressure in a patient.
It is well known in the art to measure the circulatory pressure or blood pressure of a patient by inserting a catheter into an artery of the patient, the catheter having been prefilled with physiological saline solution. The incompressible saline solution transmits pressure waves generated by the heart and modified by other portions of the circulatory system, to a transducer which is coupled to the patient's circulatory system by the column of saline solution.
It has, however, been found in the past that inaccurate readings can be generated by the use of such a system. In particular, it has been found that compliant portions of the combined patient and measurement system which include the elastic walls of the patient's circulatory system, as well as the compliant or elastic walls of the pressure measuring apparatus cause the combination of the patient's circulatory system and the pressure measuring apparatus to have one or more resonant modes. As a result of the resonant modes, circulatory pressure waves having components at or near the resonant frequencies of the system tend to be distorted in that the amplitudes of the resonant frequency components rapidly increase within the system. This often causes the health care professional using the pressure measuring system as a diagnostic tool to misread the pressure within the system and hence to misdiagnose the condition of the patient. This problem is often referred to as "ringing" or "harmonic ringing".
A number of damping devices have been proposed or built to solve the ringing problem. U.S. Pat. No. 4,335,729 to Reynolds, et al. for APPARATUS AND METHOD FOR SUPPRESSING RESONANCE AND AN ELECTROMANOMETRY SYSTEM teaches one such damping device. Details of the damping device may best be seen in FIGS. 5 and 6 of Reynolds, et al. wherein a damper having a resistance pathway which is connected in parallel to a patient-transducer circuit is employed. The damper also has a gas filled cap in communication with the resistance pathway. It should be noted, however, in order to "tune out" the resonant frequencies of the monitoring system, it is necessary to rotate the enlarged head 132, thereby effectively changing the hydraulic or pneumatic resistance of the resistance passageway. Although this would seem to be an adequate solution, it has been found that, in practice, health care professionals do not have sufficient time to tune such a system to attenuate the resonant modes of a particular patient and pressure measuring apparatus configuration due to the necessity to repeatedly change the effective resistance of the resistance channel while observing the results on a monitor.
A similar adjustable system is disclosed in U.S. Pat. No. 4,431,009 to Marino, Jr., et al. for APPARATUS FOR MEASURING BLOOD PRESSURE. The damping device may best be seen in the sectional view of FIG. 2 wherein the damping device is connected in series between the patient and the pressure transducer. Adjustment of the valve needle 48 varies the hydraulic resistance of the patient-transducer circuit thereby changing the frequency to be attenuated by the damper. This system, however, suffers from the same faults as the Reynolds, et al. system due to the fact that it also must be adjusted manually after having been connected to a patient to remove resonant mode artifacts.
A different approach is taken in U.S. Pat. No. 4,517,844 to Powell, for FLUID DAMPING DEVICE FOR A COMPLIANT SYSTEM. In the Powell system a fixed resistance path 106 is connected in parallel to the primary patient-transducer path. A compliant gas cavity or parallel capacitance device 94 is threadedly connected to the resistance path. Powell also discloses a damping device whereby the patient-transducer system may be tuned by interchanging a plurality of compliant gas cavities of various sizes in order to effect tuning of the system. This damping device would also seem to suffer from the problems of the other prior art systems in that the active intervention of a health care professional would be needed to tune out the resonant modes of the system by interchanging removable parallel capacitances. In addition, it is clear that if the pressure measuring system had been connected to a patient for tuning, disconnection of the compliant gas cavity from the pressure measuring system followed by reconnection might result in significant risk of contaminating the saline solution with air bubbles, bacteria, pyrogens, viruses and the like and causing an infection in the patient.
Accordingly, what is needed is an improved damping device which effectively damps out resonant frequencies of pressure waves in a physiological pressure monitor, but which need not be adjusted by the health care professional in order to remove resonant frequencies after being connected to a particular patient.